This invention relates to a SOI substrate having an embedded oxide layer disposed in the neighborhood of the surface of a silicon substrate and a silicon layer formed thereon (hereinafter referred to as a silicon-on-insulator (SOI) layer) and a method for the production thereof.
As SOI substrates having a single crystal silicon layer formed on such an insulator as silicon oxide, a bonded wafer and a SIMOX (separation by implanted oxygen) wafer have been mainly known. The bonded wafer is obtained by causing adhesion of two single crystal silicon wafers across an oxide film and converting either of the two wafers into a thin film. The SIMOX wafer is obtained by introducing oxygen ions into a single crystal silicon substrate by the injection of oxygen ions and causing a chemical reaction between the oxygen ions and the silicon atoms by an annealing treatment to be subsequently performed thereby giving rise to an embedded oxide film therein.
The metal-oxide-semiconductor field effect transistor (MOSFET) formed in the SOI layer of the SOI substrate possesses high resistance to radiation and to latch-up, manifests high reliability, represses the short channel effect brought by miniaturization of device, and permits an operation at a low power consumption. Thus, the SOI substrate promises as a high-performance semiconductor substrate for use in the MOS-LSI of the next generation.
(1) The MOSFET which is the basic component of a MOS-LSI is disabled to fulfill the function of a transistor when the dielectric withstanding of the oxide film under the gate electrode is not retained. Then, the whole LSI is no longer capable of retaining a normal function. As the quality of the semiconductor substrate used in the MOS-LSI, therefore, the withstanding voltage of the gate oxide film of the MOSEFT formed on the semiconductor substrate is required to secure in a fully satisfactory yield throughout the entire surface of the wafer. It has been pointed out that in the mirror wafer produced by the ordinary Czochralski process, such as-grown defects as crystal originated particles (COP) which are introduced into a crystal during the growth thereof form a factor deterioration of the withstanding voltage of the gate oxide film. The mirror wafers, therefore, have been required to reduce the density of such as-grown defects and repress the generation of poor insulation in the gate oxide film.
As regards the SOI substrate, the conditions for the formation of an embedded oxide film, the strength of lamination, the adhesion of extraneous matter, and the pollution with an introduced metal have attracted attention as problems and the improvements directed to the solution thereof have been promoted. Since no sufficient investigation has been conducted concerning the factors which are responsible for the degradation of the withstanding voltage of the gate oxide film of the MOSFET on the SOI substrate, however, the remedies produced to date in improving the oxide film in quality enough for alleviating the deficiency of the oxide film in withstanding voltage have been insufficient.
Further, the SOI substrate used for the MOS-LSI is required to avoid causing incorrect focusing at the step of lithography used in the process for production of LSI, generating etching trouble due to uneven etching at the step of etching, or entailing such a defect as to form a cause for inferior film deposition.
(2) The MOS-LSI manufactured on the SOI substrate has the device forming region thereof electrically insulated from the substrate proper through the medium of the embedded oxide layer which is an insulator. It is, therefore, capable of improving the resistance to radiation and the resistance to latch-up mentioned above and realizing such outstanding characteristics as the ability to operate with a low power consumption. For the realization of such outstanding characteristics, therefore, the MOS-LSI is required to secure the insulating performance of the embedded oxide layer in a sufficient yield throughout the entire surface of the wafer. Such as-grown defects as, for example, crystal originated particles (COP) which are introduced into a crystal during the growth thereof are present in the mirror wafer which is produced by the ordinary Czochralski process. It has been pointed out that these defects result in degrading the quality of the oxide layer as by impairing the quality of an isolation film, i.e. LOCOS (local oxidation of silicon) film. The mirror wafer, therefore, is required to reduce the density of such as-grown defects and repress the occurrence of inferior insulation of a varying oxide layer.
As regards the SOI substrate, the conditions for the formation of an embedded oxide film, the strength of lamination, the adhesion of extraneous matter, and the pollution with an introduced metal have attracted attention as problems and the improvements directed to the solution thereof have been promoted. The effect of a crystal defect on the embedded oxide layer, however, has not been studied satisfactorily.
This invention, therefore, is aimed at reducing such disadvantages and providing a SOI substrate of high quality for use in a high-performance LSI and a method therefor.
(1) We have found that the as-grown defects present in a crystal prior to the formation of a SOI structure are deformed into visible pits during the process of forming the SOI structure and consequently suffered to exert adverse effects on the device characteristics.
(2) We have found that the as-grown defects present in a crystal prior to the formation of a SOI structure exert an adverse effect on the injected oxygen profile in the case of the SIMOX process or generate defects in the surface oxide layer in the case of the lamination process and eventually give rise to defects in the embedded oxide layer in the SOI structure.
We have invented a method for preventing these adverse effects. To be specific, this invention concerns a SOI substrate intended for solving the problems mentioned above and a method for the production thereof, which comprises the following means.
According to this invention, there can be provided a SOI substrate other than SIMOX allowing the presence of an embedded oxide film in a silicon single crystal substrate and characterized by satisfying at least either of the following conditions:
(a) The density of pit-like defects observed from the surface of the SOI layer is not more than 5 cmxe2x88x922 or
(b) The density of pinhole defects in the embedded oxide layer is less than one piece/cm2.
Further, according to this invention, there can be provided a SIMOX substrate characterized in that the density of pit-like defects observed from the surface of a SOI layer is not more than 5 cmxe2x88x922.
In addition, according to this invention, there can be provided a method for the production of a SOI substrate characterized by forming a SOI structure by using at least either of the following silicon single crystal substrates:
(a) A silicon single crystal substrate wherein the region to be involved in the formation of the SOI structure has a density of defects of not more than 1xc3x97105 cmxe2x88x923 or
(b) A silicon single crystal substrate wherein voids and/or COP measuring not less than 0.1 xcexcm as reduced to diameter are absent at least in the region from the surface to a depth for forming an embedded oxide layer.